1. Field of the Invention
This invention relates to a system for and method of treating a malfunctioning heart and, more particularly, to such a system and method which effects cardioversion/defibrillation in response to sensing a heart malfunction. The invention provides for the cardioverting/defibrillation of a malfunctioning heart as well as the possibility of overcoming a tachycardia manifestation without resorting to either cardioverting or defibrillating the heart.
2. Description of the Prior Art
In recent years, substantial progress has been made in pacemakers and in the development of cardioverting/defibrillating techniques for effectively treating various heart disorders and arrhythmias. Past efforts have resulted in the development of implantable electronic pacemakers and standby cardioverters-defibrillators which, in response to the detection of an abnormal cardiac rhythm, discharge sufficient energy via electrodes connected to the heart to depolarize and restore it to normal cardiac rhythm. An early example of this cardioverting/defibrillating technique is disclosed in U.S. Pat. No. 3,942,536 of Mirowski et al., the technique involving responses to a sensed peak right ventricular systolic pressure dropping to a fixed predetermined threshold level. This known technique did not involve mean pressure changes in either direction from a baseline.
Efforts have also been directed toward developing techniques for reliably monitoring heart activity in order to determine whether cardioversion/defibrillation are desirable or necessary. Such techniques include monitoring ventricular rate or determining the presence of fibrillation on the basis of a probability density function (PDF). A system using the PDF technique statistically compares the location of points of a cardiac waveform with the expected locations of points of the normal waveform. When the waveform becomes irregular, as measured by its probability density function, an abnormal cardiac function is suggested. The latter technique is described in
U.S. Pat. Nos. 4,184,493 and 4,202,340 both of Langer et al.
A more recent system, as disclosed in U.S. Pat. No. 4,475,551 of Langer et al. utilizes both the PDF technique to determine the presence of an abnormal cardiac rhythm and a heart rate sensing circuit for distinguishing between ventricular fibrillation and high rate tachycardia (the latter being indicated by a heart rate above a predetermined minimum threshold), on the one hand, and normal sinus rhythm or a low rate tachycardia (indicated by a heart rate falling below a pre-determined minimum threshold), on the other hand.
Still further, research in this area has resulted in the development of a heart rate detector system which accurately measures heart rate from a variety of different electrocardiogram (ECG) signal shapes. One such system is disclosed in U.S. Pat. No. 4,393,877 of Imran et al.
Despite these past efforts and the level of achievement prevalent among prior art systems, there are potential difficulties and drawbacks which may be experienced with such devices.
Currently antitachycardia systems detect arrhythmias primarily by sensing rate and perform inadequately in the differentiation of hemodynamically stable from unstable rhythms. These devices, for example, may fire during a stable supraventricular tachycardia (SVT) inflicting pain and wasting energy; damage to the heart may result.
A commonly used implantable antitachycardia device is the automatic implantable cardioverter-defibrillators (AICD) which is commercially available under the model designations 1500, 1510 and 1520 from Cardiac Pacemakers, Inc. whose address is: 4100 North Hamlin Avenue, St. Paul, Minn. 55164. These devices continuously monitor myocardial electrical activity, detecting ventricular tachycardia (VT) and ventricular fibrillation (VF), and delivering a shock to the myocardium to terminate the arrhythmia. The AICD has been shown to reduce the mortality rate in patients with malignant arrhythmias with initial studies at Johns Hopkins Hospital and Stanford Medical Center demonstrating a 50 percent decrease in the anticipated total incidence of death, as reported by Mirowski et al., "Recent Clinical Experience with the Automatic Implantable Cardioverter-Defibrillator", Medical Instrumentation, Vol. 20, pages 285-291 (1986). Arrhythmias are detected by (1) a rate (R wave) sensor and (2) a probability density function (PDF) which defines the fraction of time spent by the differentiated electrocardiogram between two amplitude limits located near zero potential. Presently, the functional window of the PDF is wide to permit the detection of both VT and VF, and therefore, this device functions essentially as a rate-only sensing system. As reported by Mirowski, "The Automatic Implantable Cardioverter-Defibrillator: An Overview", JACC, Vol. 6, No. 2, pages 461-466, (August, 1985), when an arrhythmia fulfills either the rate or PDF criteria, the device delivers Schuder's truncated exponential pulse of 25 Joules some 17 seconds after the onset of the arrhythmia. The device can recycle as many as three times if the previous discharge is ineffective with the strength of the second, third and fourth pulses being increased to 30 Joules. After the fourth discharge, approximately 35 seconds of nonfibrillating rhythm are required to reset the device. The Mirowski et al., supra, and the Mirowski, supra publications set out, in summary form, background material relating to the defibrillating/cardioverting arts against which the present invention was made.
In addition to the standard automatic implantable cardioverter-defibrillator characterized by the above-noted, dual detection algorithm, a variant of the device which features a sensing system that relies only on the analysis of heart rate is also available. This "rate-only" version of the known cardioverter-defibrillator preferred by some investigators, is more sensitive than the dual detection version unit and theoretically less likely to miss ventricular tachycardias with narrow QRS complexes. It is believed that the "rate-only" system, on the other hand, may be too sensitive, delivering cardioverting/defibrillating pulses too often or too soon, no hemodynamic parameter having been taken into consideration.
One problem with current systems is that they function primarily as a rate-only sensing systems and may fire for nonmalignant as well as malignant tachycardias. These firings are not benign; potentially endangering myocardium, wasting energy and inflicting pain on the conscious patient, all distinct shortcomings and disadvantages.
The principal object of the present invention is to provide a system for cardioverting/defibrillating which avoids unnecessary firings, thereby reducing the danger to the myocardium, saving energy and avoiding pain.
Another object of the present invention is to provide an implantable system for cardioverting/defibrillating which avoids unnecessary firings, thereby reducing the danger to the myocardium, saving energy and avoiding pain.
A further object of the present invention is to provide a system for cardioverting/defibrillating which is hemodynamically responsive to change in mean pressure from a baseline.
An additional object of the present invention is to provide a system for cardioverting/defibrillating which is hemodynamically responsive to change in mean pressure from a baseline and to rate criteria.
Yet another object of the present invention is to provide a method of cardioverting/defibrillating which may be advantageously carried out using a cardioverter-defibrillator constructed in accordance with the present invention.
Yet a further object of the present invention is to provide a method of cardioverting/defibrillating which avoids unnecessary firings thereby reducing the danger to the myocardium, saving energy and avoiding pain.
In accordance with preferred embodiments of the present invention, new sensing algorithms are proposed using hemodynamic or both hemodynamic and rate criteria, the latter being taken in series or parallel. The series configuration algorithm could be effected by detecting rate with an intracardiac, extracardiac, or body-surface R-wave sensor. When rate exceeds the programmed cut-off value, at least one hemodynamic parameter, such as mean right atrial pressure (MRAP), mean right ventricular pressure (MRVP), mean central venous pressure (MCVP) or mean arterial pressure (MAP) departures from a baseline would be monitored. Mean left atrial pressure (MLAP) or mean left ventricular pressure (MLVP) may also be suitable as one or another of the hemodynamic baseline parameters from which changes may be monitored. If mean right arterial pressure (MRAP) or mean right ventricular pressure (MRVP) or mean central venous pressure (MCVP) increases from respective baseline MRAP or MRVP or MCVP baselines within a time period of predetermined duration, indicating hemodynamic compromise, the system would fire. If mean left atrial pressure (MLAP) or mean left ventricular pressure (MLVP) increases respectively from respective baseline MLAP or baseline MLVP within a time period of predetermined duration indicating hemodynamic compromise, the system would fire. If mean arterial pressure (MAP) decreases from baseline MAP beyond a predetermined magnitude indicating hemodynamic compromise the system would fire. If the respective pressure changes were less than the respective predetermined magnitudes, pressures would be monitored to determine if respective changes from the respective mean levels take place, as long as the rate criteria is satisfied. A parallel configuration algorithm in which rate and hemodynamic criteria function simultaneously is also proposed; however, continuous pressure change determination would probably be less energy efficient. Either configuration of algorithm could be adapted to a single catheter consisting of a pressure transducer in either the right atrium or right ventricle and an R-wave sensing electrode or pair of electrodes at the catheter tip in the right ventricle. The hemodynamic information derived from an arterial line, Swan-Ganz catheter (already present in the intensive/cardiac care unit patients), or even an automated mechanical blood pressure cuff could be integrated together with the electrocardiogram to provide a temporary automatic antitachycardia system. Cardioversion-defibrillation could be administered using externally applied patches. Even a noninvasive hemodynamically responsive antitachycardia system is potentially feasible using doppler technology for pressure measurements. The PDF (narrow window of function) and the rate/pressure sensing algorithm could be used simultaneously such that if the rate/pressure criteria are satisfied (indicating hemodynamically significant SVT or VT) the device cardioverters and if the PDF criteria is satisfied indicating (VF) defibrillation results. This pulse delivery system could also be incorporated into a single catheter.
It is to be appreciated that when the pressure criteria is not met, but the rate criteria indicates tachycardia is present, an antitachycardia pacemaker could be enabled in an effort to correct the malfunction.
MAP is an excellent parameter but accurate continuous measurement requires an indwelling arterial catheter or transducer which over time is prone to infection and thrombus formation (with the potential for systemic embolic events). MRAP and MRVP appear to relate useful information regarding the hemodynamic state of the particular arrhythmia. If tricuspid stenosis were present, MRVP would probably be more reliable than MRAP. Preliminary observations in the canine model suggest that changes as small as 3 mmHg for MRAP and MRVP and as small as 15 mmHg for MAP are significant and can be used in carrying out the present invention.
The rate/pressure sensing algorithms could also help integrate a cardioverter-defibrillator with an antitachycardia pacemaker. The hemodynamic function would determine which of these devices to engage. For example, when a hemodynamically significant tachycardia is detected the cardioverter-defibrillator would be used to terminate the arrhythmia. When a hemodynamically stable tachycardia is sensed the antitachycardia pacemaker would attempt to terminate the arrhythmia using such methods as overdrive, burst, or extra stimulus pacing, incremental or decremental scanning, or ultra-high frequency stimulation. If the tachycardia was accelerated, this would be detected by the rate/pressure sensing algorithm and cardioverted or defibrillated. With a pacemaker present, a bradycardia failsafe could be built into the system.
The adaptation of a hemodynamic parameter to the sensing system of antitachycardia devices appears to be a logical improvement to its present function. MRAP and MRVP are easily measured parameters (via the transvenous route) and appear to relate important hemodynamic information. MAP is an easily measured parameter in the intensive/cardiac care unit setting and could be integrated together with the electrocardiogram to form a temporary automatic antitachycardia system. A rate/pressure sensing algorithm, designed either in series or parallel, could be integrated with the PDF system such that hemodynamically significant SVT, VT, and VF would be detected. The rate/pressure sensing algorithm could also be applied to a combined cardioverter-defibrillator and antitachycardia pacemaker.
From one vantage point, the invention can be seen as a system for treating a malfunctioning heart which includes storage means for storing electrical energy and electrode means for electrically coupling the storage means to the heart. Pressure responsive means sense pressure at a site in a circulatory system. Means provide a first signal representative of fixed baseline pressure and means responsive to output from the sensing means develop a second signal representing mean current pressure over a period of given length. Means respond to output from the means for providing the first signal and output from the means for developing the second signal for charging and enabling discharge of the electrical energy stored by the storage means across the electrode means and into the heart, upon change in the mean current pressure of at least a predetermined amount from the representative fixed baseline pressure.
The invention can also be seen as a system for treating a malfunctioning heart which has electrical means for sensing heart rate and which produces a first control signal upon the heart rate exceeding a predetermined rate. Pressure responsive sensing means sense pressure at a site in a circulatory system. Means provide a first signal representative of fixed baseline pressure and means responsive to output from the sensing means develop a pressure-related second signal representing mean current pressure at the site over a period of given length and produce a second control signal indicative of the mean current pressure at the site departing from fixed baseline pressure by at least a predetermined amount. Controllable antitachycardia pacemaking means are provided for supplying pacing signals to the heart. Controllable cardioverting/defibrillating means, including storage means for storing electrical energy and electrode means, are provided for applying electrical energy to the heart to cardiovert or to defibrillate same. Control circuit means responsive to the first control signal and to the second control signal enable the antitachycardia pacemaking means in response to presence of the first control signal and contemporaneous absence of the second control signal and enable the cardioverting/defibrillating means in response to contemporaneous presence of both the first control signal and the second control signal. Electrical energy stored by the storage means may be discharged, via the electrode means, into the heart.
From another vantage point, the invention can be viewed as a system for treating a malfunctioning heart having means for providing cardioverting/defibrillating electrical energy, pressure responsive sensing means for sensing pressure at a site in a circulatory system and means for providing a fixed signal representative of fixed baseline pressure. Means responsive to output from the pressure responsive sensing means develop a further signal representing mean pressure over a period of given length. Means responsive to the fixed signal and to the further signal deliver the cardioverting/defibrillating electrical energy into the heart upon occurrence of departure of the further signal from the fixed signal by at least a predetermined magnitude indicative of hemodynamic compromise.
The invention may be seen as a system for treating a malfunctioning heart provided with electrical means for sensing heart rate to produce a first control signal whenever the rate exceeds a predetermined rate. Pressure responsive means sense pressure at a site in a circulatory system. Means are operatively arranged to provide a fixed signal representative of fixed baseline pressure. Means responsive to output from the pressure responsive means develop a further signal representing mean pressure over a period of given length. Means respond to the first signal and to the further signal to develop a second control signal upon departure of the further signal from the first signal by at least a predetermined magnitude. Controllable antitachycardia pacemaking means are provided to supply pacing signals to the heart. Means are operatively arranged to produce controllable cardioverting/defibrillating electrical energy. Control circuit means respond to the first control signal and to the second control signal to enable the antitachycardia pacemaking means upon presence of the first control signal and absence of the second control signal and to enable the means for producing the cardioverting/defibrillating electrical energy upon contemporaneous presence of the first control signal and the second control signal.
The invention also can be viewed as being in a system for treating a malfunctioning heart of a patient having means for delivering electrical energy to the heart and pressure responsive means for sensing pressure at a site in a circulatory system. Means provide a first signal representative of fixed baseline pressure and means, coupled to the pressure responsive means and responsive to output therefrom, produce a second signal representing mean pressure over a period of given length. Means responsive to the first signal and to the second signal produce a control signal upon occurrence of the second signal departing from the first signal by at least a given magnitude. Means responsive to the control signal enable the means for delivering electrical energy to the heart.
The invention can be also seen as a system for treating a malfunctioning heart of a patient having pressure responsive means for sensing pressure at a site in a circulatory system and means for providing a first signal representing fixed baseline pressure. Means coupled to the pressure responsive means respond to output therefrom to produce a second signal representing mean pressure over a period of given length. Means responsive to the first signal and to the second signal produce a first control signal upon occurrence of the second signal departing from the first signal by at least a given magnitude. Means responsive to heart rate produce a second control signal upon heart rate exceeding a given rate. Means respond to at least the first control signal and to the second control signal and deliver electrical energy to the heart.
In its method aspect, the invention is seen as a method of treating a malfunctioning heart which includes step of sensing pressure at a site in a circulatory system. The salient steps include providing a representation of fixed baselike pressure, determining mean current pressure from the sensed pressure at the site over a period of given length, and delivering cardioverting/defibrillating electrical energy to the heart in response to change of at least a predetermined magnitude in the mean current pressure from fixed baseline pressure.
In its method aspect, the invention can be seen as being in a method of treating a malfunctioning heart of a patient which includes sensing change in pressure at a site in a circulatory system and delivering to the patient electrical energy. The salient steps include providing a representation of fixed baseline pressure and determining mean current pressure from the sensed pressure over a period of given length. The step of delivering electrical energy is taken in response to a difference of at least predetermined magnitude between mean current pressure and fixed baseline pressure.
The method many include determining heart rate, the step of delivering to the heart electrical energy being taken, in this case, only upon the heart rate exceeding a given rate and the at least predetermined magnitude between mean current pressure and base line pressure prevails.
The novel features that are considered characteristic of the invention in its method and system aspects are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with other objects and advantages thereof is to be understood from the following description of illustrative embodiments, when read in conjunction with the accompanying drawings, wherein like reference numerals refer to like components.